Compare sampled values

From what I’ve been able to find, it appears that the right hand side of a match has to be a constant. Is there any way to compare one sampled value against another? Specifically I’d like to send a configurable % of certain requests to a different backend, so I’d like to write something like this:

acl use_alt_backend var(txn.configured_percent) gt %[rand(100)]

where txn.configured_percent was looked up from a map file with map_str_int using a value extracted from the path.